Mass-loss Mechanism Research Articles

Effects of calving and submarine melting on steady states and stability of buttressed marine ice sheets

AbstractMass loss from ice shelves is a strong control on grounding-line dynamics. Here we investigate how calving and submarine melt parameterizations affect steady-state grounding-line positions and their stability. Our results indicate that different calving laws with the same melt parameterization result in more diverse steady-state ice-sheet configurations than different melt parameterizations with the same calving law. We show that the backstress at the grounding line depends on the integrated ice-shelf mass flux. Consequently, ice shelves are most sensitive to high melt rates in the vicinity of their grounding lines. For the same shelf-averaged melt rates, different melt parameterizations can lead to very different ice-shelf configurations and grounding-line positions. If the melt rate depends on the slope of the ice-shelf draft, then the positive feedback between increased melting and steepening of the slope can lead to singular melt rates at the ice-shelf front, producing an apparent lower limit of the shelf front thickness as the ice thickness vanishes over a small boundary layer. Our results illustrate that the evolution of marine ice sheets is highly dependent on ice-shelf mass loss mechanisms, and that existing parameterizations can lead to a wide range of modelled grounding-line behaviours.

Dynamic modeling of Pt degradation and mitigation strategies in polymer electrolyte membrane fuel cells

The durability issue of polymer electrolyte membrane fuel cells must be resolved before their widespread commercialization in transportation. A central issue in fuel cell durability is the loss of Pt electrochemical active surface area (ECSA) during automotive conditions, which is induced primarily by the Pt mass loss and Pt particle growth mechanisms. A comprehensive Pt degradation model to study the evolution of the ECSA distribution, induced mechanisms and mitigation strategies is developed. After validation with experimental results, the importance of the Pt mass loss and particle growth mechanisms is studied. Results reveal that the Pt mass loss mechanism occurs primarily near the proton-exchange-membrane/catalyst-layer (PEM/CL) interface and becomes more critical with improved electrode potential. The ECSA loss by the particle growth (Ostwald ripening) mechanism is, for the most part, uniformly distributed along the thickness direction, whereas it is suppressed near the PEM/CL interface by the Pt mass loss mechanism. The model also studies possible mitigation strategies for ECSA loss by suppressing the mass loss mechanism. Both the graded PEM structure and reduced Pt 2+ ion diffusivity can help mitigate the ECSA loss process. • Modeling of Pt degradation in the membrane electrode assembly is developed. • The model distinguishes Pt ECSA loss by mass loss and particle growth types. • Pt mass loss mechanism distributes nonuniformly and increases with cell voltage. • Pt growth mechanism distributes uniformly and increases with voltage cycling. • The model proposes mitigation strategies for ECSA loss by reducing Pt mass loss.

The California-Kepler Survey. X. The Radius Gap as a Function of Stellar Mass, Metallicity, and Age

Abstract In 2017, the California-Kepler Survey (CKS) published its first data release (DR1) of high-resolution optical spectra of 1305 planet hosts. Refined CKS planet radii revealed that small planets are bifurcated into two distinct populations, super-Earths (smaller than 1.5 R ⊕) and sub-Neptunes (between 2.0 and 4.0 R ⊕), with few planets in between (the “radius gap”). Several theoretical models of the radius gap predict variation with stellar mass, but testing these predictions is challenging with CKS DR1 due to its limited M ⋆ range of 0.8–1.4 M ⊙. Here we present CKS DR2 with 411 additional spectra and derived properties focusing on stars of 0.5–0.8 M ⊙. We found that the radius gap follows R p ∝ P m with m = −0.10 ± 0.03, consistent with predictions of X-ray and ultraviolet- and core-powered mass-loss mechanisms. We found no evidence that m varies with M ⋆. We observed a correlation between the average sub-Neptune size and M ⋆. Over 0.5–1.4 M ⊙, the average sub-Neptune grows from 2.1 to 2.6 R ⊕, following R p ∝ M ⋆ α with α = 0.25 ± 0.03. In contrast, there is no detectable change for super-Earths. These M ⋆–R p trends suggest that protoplanetary disks can efficiently produce cores up to a threshold mass of M c , which grows linearly with stellar mass according to M c ≈ 10 M ⊕(M ⋆/M ⊙). There is no significant correlation between sub-Neptune size and stellar metallicity (over −0.5 to +0.5 dex), suggesting a weak relationship between planet envelope opacity and stellar metallicity. Finally, there is no significant variation in sub-Neptune size with stellar age (over 1–10 Gyr), which suggests that the majority of envelope contraction concludes after ∼1 Gyr.

Pair-instability Mass Loss for Top-down Compact Object Mass Calculations

Abstract Population synthesis relies on semi-analytic formulae to determine masses of compact objects from the (helium or carbon-oxygen) cores of collapsing stars. Such formulae are combined across mass ranges that span different explosion mechanisms, potentialy introducing artificial features in the compact object mass distribution. Such artifacts impair the interpretation of gravitational-wave observations. We propose a “top-down” remnant mass prescription where we remove mass from the star for each possible mass-loss mechanism, instead of relying on the fallback onto a “proto-compact-object” to get the final mass. For one of these mass-loss mechanisms, we fit the metallicity-dependent mass lost to pulsational-pair instability supernovae from numerical simulations. By imposing no mass loss in the absence of pulses, our approach recovers the existing compact object masses at the low mass end and ensures continuity across the core-collapse/pulsational-pair-instability regime.

Resurrection of Nonthermal Emissions from Type Ib/c Supernova Remnants

Abstract Supernova remnants (SNRs) are important objects in investigating the links among supernova (SN) explosion mechanism(s), progenitor stars, and cosmic-ray acceleration. Nonthermal emission from SNRs is an effective and promising tool for probing their surrounding circumstellar media (CSM) and, in turn, the stellar evolution and mass-loss mechanism(s) of massive stars. In this work, we calculate the time evolution of broadband nonthermal emissions from Type Ib/c SNRs, whose CSM structures are derived from the mass-loss history of their progenitors. Our results predict that Type Ib/c SNRs make a transition of brightness in radio and γ-ray bands from an undetectable dark for a certain period to a rebrightening phase. This transition originates from their inhomogeneous CSM structures in which the SNRs are embedded within a low-density wind cavity surrounded by a high-density wind shell and the ambient interstellar medium (ISM). The “resurrection” in nonthermal luminosity happens at an age of ∼1000 yr old for a Wolf-Rayet star progenitor evolved within a typical ISM density. Combining with the results of Type II SNR evolution recently reported by Yasuda et al., this result sheds light on a comprehensive understanding of nonthermal emissions from SNRs with different SN progenitor types and ages, which is made possible for the first time by the incorporation of realistic mass-loss histories of the progenitors.

Episodic Gaseous Outflows and Mass Loss from Red Supergiants

Abstract The red hypergiant VY CMa and the more typical red supergiant (RSG) Betelgeuse provide clear observational evidence for discrete, directed gaseous outflows in their optical and infrared imaging, spectra, and light curves. In the very luminous VY CMa, mass-loss estimates from the infrared-bright knots and clumps not only dominate its measured overall mass loss, but explain it. In the less luminous Betelgeuse, similar mass estimates of its circumstellar condensations show that they contribute significantly to its measured mass-loss rate. We present new measurements for both stars and discuss additional evidence for gaseous ejections in other RSGs. Gaseous outflows are the dominant mechanism for the most luminous RSGs and an important contributor to the more typical RSGs like Betelgeuse. We conclude that gaseous outflows, related to magnetic fields and surface activity, comparable to coronal mass ejections, are a major contributor to mass loss from RSGs and the missing component in discussions of their mass-loss mechanism.

The physics governing the upper truncation mass of the globular cluster mass function

ABSTRACT The mass function of globular cluster (GC) populations is a fundamental observable that encodes the physical conditions under which these massive stellar clusters formed and evolved. The high-mass end of star cluster mass functions are commonly described using a Schechter function, with an exponential truncation mass Mc, *. For the GC mass functions in the Virgo galaxy cluster, this truncation mass increases with galaxy mass (M*). In this paper, we fit Schechter mass functions to the GCs in the most massive galaxy group ($M_{\mathrm{200}} = 5.14 \times 10^{13} \, {\rm M}_{\odot }$) in the E-MOSAICS simulations. The fiducial cluster formation model in E-MOSAICS reproduces the observed trend of Mc, * with M* for the Virgo cluster. We therefore examine the origin of the relation by fitting Mc, * as a function of galaxy mass, with and without accounting for mass loss by two-body relaxation, tidal shocks and/or dynamical friction. In the absence of these mass-loss mechanisms, the Mc, *-M* relation is flat above $M_* \gt 10^{10}\, {\rm M}_{\odot }$. It is therefore the disruption of high-mass GCs in galaxies with $M_{*}\sim 10^{10} \, {\rm M}_{\odot }$ that lowers the Mc, * in these galaxies. High-mass GCs are able to survive in more massive galaxies, since there are more mergers to facilitate their redistribution to less-dense environments. The Mc, * − M* relation is therefore a consequence of both the formation conditions of massive star clusters and their environmentally dependent disruption mechanisms.

A Laboratory-driven Multiscale Investigation of X-Ray Induced Mass Loss and Photochemical Evolution in Cosmic Carbon and Silicate Dust

Abstract We present the results of an integrated laboratory and modeling investigation into the impact of stellar X-rays on cosmic dust. Carbonaceous grains were prepared in a cooled (<200 K) supersonic expansion from aromatic molecular precursors, and were later irradiated with 970 eV X-rays. Silicate (enstatite) grains were prepared via laser ablation, thermally annealed, and later irradiated with 500 eV X-rays. Infrared spectra of the 3.4 μm band of the carbon sample prepared with benzene revealed 84% ± 5% band area loss for an X-ray dose of 5.2 ×1023 eV.cm−2. Infrared spectra of the 8–12 μm Si–O band of the silicate sample revealed band area loss up to 63% ± 5% for doses of 2.3 × 1023 eV.cm−2. A hybrid Monte Carlo particle trajectory approach was used to model the impact of X-rays and ensuing photoelectrons, Auger and collisionally ionized electrons through the bulk. As a result of X-ray ionization and ensuing Coulomb explosions on surface molecules, the calculated mass loss is 60% for the carbonaceous sample and 46% for the silicate sample, within a factor of 2 of the IR band loss, supporting an X-ray induced mass-loss mechanism. We apply the laboratory X-ray destruction rates to estimate the lifetimes of dust grains in protoplanetary disks surrounding 1 M ⊙ and 0.1 M ⊙ G and M stars. In both cases, X-ray destruction timescales are short (a few million years) at the disk surface, but are found to be much longer than typical disk lifetimes (≳10 Myr) over the disk bulk.

Abrupt climate changes in the last two deglaciations simulated with different Northern ice sheet discharge and insolation

There were significant differences between the last two deglaciations, particularly in Atlantic Meridional Overturning Circulation (AMOC) and Antarctic warming in the deglaciations and the following interglacials. Here, we present transient simulations of deglaciation using a coupled atmosphere–ocean general circulation model for the last two deglaciations focusing on the impact of ice sheet discharge on climate changes associated with the AMOC in the first part, and the sensitivity studies using a Northern Hemisphere ice sheet model in the second part. We show that a set of abrupt climate changes of the last deglaciation, including Bolling–Allerod warming, the Younger Dryas, and onset of the Holocene were simulated with gradual changes of both ice sheet discharge and radiative forcing. On the other hand, penultimate deglaciation, with the abrupt climate change only at the beginning of the last interglacial was simulated when the ice sheet discharge was greater than in the last deglaciation by a factor of 1.5. The results, together with Northern Hemisphere ice sheet model experiments suggest the importance of the transient climate and AMOC responses to the different orbital forcing conditions of the last two deglaciations, through the mechanisms of mass loss of the Northern Hemisphere ice sheet and meltwater influx to the ocean.

Experimental study of the mechanical properties of basalt fibre-reinforced concrete at elevated temperatures

Generally, achieving a proper mixing ratio of basalt fibre in concrete can improve the mechanical properties of concrete and enhance its anti-corrosion ability. Although basalt fibre exhibits excellent resistance to high temperatures, the mechanical behaviour of basalt fibre-reinforced concrete (BFRC) exposed to high temperatures is still unclear. In addition to the basic compressive and tensile strength testing for ordinary concrete and BFRC, the core temperature, mass loss, mechanical properties and synergistic mechanism of basalt fibre and concrete matrix under different temperatures were evaluated. The results show that BFRC provides excellent thermal insulation and can mitigate the concrete spalling phenomenon. Therefore, using basalt fibre can effectively improve the mechanical properties of concrete exposed to high temperatures.

The Impact of Shocks on the Vertical Structure of Eccentric Disks

Abstract Accretion disks whose matter follows eccentric orbits can arise in multiple astrophysical situations. Unlike circular orbit disks, the vertical gravity in eccentric disks varies around the orbit. In this paper, we investigate some of the dynamical effects of this varying gravity on the vertical structure using 1D hydrodynamics simulations of individual gas columns assumed to be mutually noninteracting. We find that time-dependent gravitational pumping generically creates shocks near pericenter; the energy dissipated in the shocks is taken from the orbital energy. Because the kinetic energy per unit mass in vertical motion near pericenter can be large compared to the net orbital energy, the shocked gas can be heated to nearly the virial temperature, and some of it becomes unbound. These shocks affect larger fractions of the disk mass for larger eccentricity and/or disk aspect ratio. If the orbit can be maintained despite orbital energy loss, diverse initial structures evolve in only a few orbits so that they follow a limit cycle characterized by a low-entropy midplane and a much higher entropy outer layer. In favorable cases (such as the tidal disruption of stars by supermassive black holes), these effects could be a potentially important energy dissipation and mass-loss mechanism.

Molecules and Outflows in NML Cygni: New Insights from a 1 mm Spectral Line Survey

Abstract A sensitive (1σ rms ≤ 3 mK) 1 mm spectral survey (214.5–285.5 GHz) of the oxygen-rich circumstellar envelope of the red hypergiant NML Cygni (NML Cyg) has been conducted using the Sub-millimeter Telescope (SMT) of the Arizona Radio Observatory (ARO). Over 100 spectral lines were detected, arising from 17 different molecules, including the carbon-bearing species CO, HCN, HCO+, CN, and HNC; sulfur- and silicon-containing compounds H2S, SO, SO2, SiO, and SiS; and more exotic NaCl and AlO. The 1 mm spectrum of NML Cyg closely resembles that of VY Canis Majoris (VY CMa) suggesting that the chemistries of hypergiant stars are similar. The line profiles in NML Cyg consist of multiple velocity features, particularly evident in SO2 and SO. In addition to a spherical wind at the star’s systemic velocity, the spectra suggest an asymmetric, blueshifted component near V LSR = −21 ± 3 km s−1 and a collimated, redshifted component near 15 ± 3 km s−1, positioned ∼34° and ∼12°, respectively, from the line of sight. The red- and blueshifted flows appear to be randomly oriented, and likely trace sporadic mass loss events. Their LSR velocities align closely with those of 22 GHz water masers, suggesting an NE–SE orientation. The winds may also be associated with the asymmetric nebula in F555W HST images but extending to 5″ (∼600R *). NML Cyg appears to be another example of rare, massive stars with collimated, episodic ejections, analogous to Betelgeuse and VY CMa, lending support for an important new mass loss mechanism—surface activity.

Dark Age of Type II Supernova Remnants

Abstract Supernova remnants (SNRs) are important objects in terms of their connections with supernova (SN) explosion mechanism(s), progenitor stars, and cosmic-ray acceleration. Nonthermal emission from SNRs is an effective probe of the structure of their surrounding circumstellar media (CSM), which can in turn shed lights on the mechanism and history of the elusive mass loss of massive stars. In this work, we calculate the time evolution of broadband nonthermal emission from SNRs originating from Type II SNe embedded in a CSM environment linked to the mass-loss history of the progenitor. Our results predict that Type II SNRs experience a prolonged period of weak radio and γ-ray emission if they run into a spatially extended bubble of low density and high temperature created by the stellar wind during main sequence. For a typical red supergiant progenitor evolved within an average interstellar medium, this “dark age” corresponds to a range of SNR ages spanning from ∼1000 to 5000 yr old. This result suggests that a majority of Type II SNRs are too faint to be detected, which may help explain why the number of known Galactic SNRs is significantly less than what we expect from the SN rate in our Galaxy.

TOI-1749: an M dwarf with a Trio of Planets including a Near-resonant Pair

Abstract We report the discovery of one super-Earth- (TOI-1749b) and two sub-Neptune-sized planets (TOI-1749c and TOI-1749d) transiting an early M dwarf at a distance of 100 pc, which were first identified as planetary candidates using data from the TESS photometric survey. We have followed up this system from the ground by means of multiband transit photometry, adaptive optics imaging, and low-resolution spectroscopy, from which we have validated the planetary nature of the candidates. We find that TOI-1749b, c, and d have orbital periods of 2.39, 4.49, and 9.05 days, and radii of 1.4, 2.1, and 2.5 R ⊕, respectively. We also place 95% confidence upper limits on the masses of 57, 14, and 15 M ⊕ for TOI-1749b, c, and d, respectively, from transit timing variations. The periods, sizes, and tentative masses of these planets are in line with a scenario in which all three planets initially had a hydrogen envelope on top of a rocky core, and only the envelope of the innermost planet has been stripped away by photoevaporation and/or core-powered mass-loss mechanisms. These planets are similar to other planetary trios found around M dwarfs, such as TOI-175b,c,d and TOI-270b,c,d, in the sense that the outer pair has a period ratio within 1% of 2. Such a characteristic orbital configuration, in which an additional planet is located interior to a near 2:1 period-ratio pair, is relatively rare around FGK dwarfs.

Skeletal muscle wasting after a severe burn is a consequence of cachexia and sarcopenia.

Muscle wasting is common and persistent in severely burned patients, worsened by immobilization during treatment. In this review, we posit two major phenotypes of muscle wasting after severe burn, cachexia and sarcopenia, each with distinguishing characteristics to result in muscle atrophy; these characteristics are also likely present in other critically ill populations. An online search was conducted from the PubMed database and other available online resources and we manually extracted published articles in a systematic mini review. We describe the current definitions and characteristics of cachexia and sarcopenia and relate these to muscle wasting after severe burn. We then discuss these putative mechanisms of muscle atrophy in this condition. Severe burn and immobilization have distinctive patterns in mediating muscle wasting and muscle atrophy. In considering these two pathological phenotypes (cachexia and sarcopenia), we propose two independent principal causes and mechanisms of muscle mass loss after burns: (1) inflammation‐induced cachexia, leading to proteolysis and protein degradation, and (2) sarcopenia/immobility that signals inhibition of expected increases in protein synthesis in response to protein loss. Because both are present following severe burn, these should be considered independently in devising treatments. Discussing cachexia and sarcopenia as independent mechanisms of severe burn–initiated muscle wasting is explored. Recognition of these associated mechanisms will likely improve outcomes.

Fast Blue Optical Transients Due to Circumstellar Interaction and the Mysterious Supernova SN 2018gep

Abstract The discovery of SN 2018gep (ZTF 18abukavn) challenged our understanding of the late-phase evolution of massive stars and their supernovae (SNe). The fast rise in luminosity of this SN (spectroscopically classified as a broad-lined Type Ic SN) indicates that the ejecta interacts with a dense circumstellar medium (CSM), while an additional energy source such as 56Ni decay is required to explain the late-time light curve. These features hint at the explosion of a massive star with pre-SN mass loss. In this work, we examine the physical origins of rapidly evolving astrophysical transients like SN 2018gep. We investigate the wave-driven mass-loss mechanism and how it depends on model parameters such as progenitor mass and deposition energy, searching for stellar progenitor models that can reproduce the observational data. A model with an ejecta mass ∼2 M ⊙, explosion energy ∼1052 erg, a CSM of mass ∼0.3 M ⊙ and radius ∼1000 R ⊙, and a 56Ni mass ∼0.3 M ⊙ provides a good fit to the bolometric light curve. We also examine how interaction-powered light curves depend more generally on these parameters and how ejecta velocities can help break degeneracies. We find both wave-driven mass loss and mass ejection via pulsational pair instability can plausibly create the dense CSM in SN 2018gep, but we favor the latter possibility.

Slingshot prominences: a hidden mass loss mechanism

ABSTRACT Whilst ‘slingshot’ prominences have been observed on M-dwarfs, most if not all theoretical studies have focused on solar-like stars. We present an investigation into stellar prominences around rapidly rotating young M-dwarfs. We have extrapolated the magnetic field in the corona from Zeeman-Doppler maps and determined the sites of mechanical stability where prominences may form. We analyse the prominence mass that could be supported and the latitude range over which this material is distributed. We find that for these maps, much of this prominence mass may be invisible to observation – typically <1 per cent transits the stellar disc. On the rapidly rotating M-dwarf V374 Peg (Prot = 0.45 d) where prominences have been observed, we find the visible prominence mass to be around only 10 per cent of the total mass supported. The mass loss rate per unit area for prominences scales with the X-ray surface flux as $\dot{M}/A \propto \, F_\mathrm{ X}^{1.32}$ that is very close to the observationally derived value for stellar winds. This suggests that prominence ejection may contribute significantly to the overall stellar wind loss and spin-down. A planet in an equatorial orbit in the habitable zone of these stars may experience intermittent enhancements of the stellar wind due to prominence ejections. On some stars, this may occur throughout 20 per cent of the orbit.

Efficiency Mechanisms in Porous-Media Electrospray Thrusters

The overall propulsion efficiency for ion-mode electrospray thrusters has been predicted to be as high as 90%; however, experimental measurements currently fall far short of these predictions. Further complicating this is that for passively fed electrospray thrusters, the mass flow rate, which is required to obtain the propulsion efficiency and specific impulse, is not directly controlled or measured, and so this parameter is typically estimated by assuming all mass loss is due to the emitted ion current. Presented here is a detailed investigation into the efficiencies associated with a porous-media-based electrospray thruster operated in the purely ionic regime using the ionic-liquid propellant 1-ethyl-3-methylimidazolium tetrafluoroborate in both positive and negative ion emission modes. Measurements of performance metrics that affect thruster efficiency are discussed, including the transmission, angular, polydispersive, energy, and mass utilization efficiencies, in order to determine their impact on overall efficiency. The overall propulsion efficiency and specific impulse are also calculated using a variety of methods to better investigate how the assumptions made to estimate the mass flow rate affect these parameters. These results suggest that the efficiency of these devices may primarily be limited by the presence of additional mass loss mechanisms other than ion emission occurring during thruster operation.

Caught in the act: core-powered mass-loss predictions for observing atmospheric escape

ABSTRACT Past studies have demonstrated that atmospheric escape by the core-powered mass-loss mechanism can explain a multitude of observations associated with the radius valley that separates the super-Earth and sub-Neptune planet populations. Complementing such studies, in this work, we present a shortlist of planets that could be losing their atmospheres today if their evolution is indeed primarily dictated by core-powered mass-loss. We use Bayesian inference analysis on our planet evolution and mass-loss model to estimate the posteriors of the parameters that encapsulate the current state of a given planet, given their published masses, radii, and host star properties. Our models predict that the following planets could be losing their atmospheres today at a rate ≳107 g s−1 at 50 per cent confidence level: pi Men c, Kepler-60 d, Kepler-60 b, HD 86226 c, EPIC 249893012 b, Kepler-107 c, HD 219134 b, Kepler-80 e, Kepler-138 d, and GJ 9827 d. As a by-product of our Bayesian inference analysis, we were also able to identify planets that most-likely harbour either secondary atmospheres abundant with high mean-molecular weight species, low-density interiors abundant with ices, or both. The planets belonging to this second category are WASP-47 e, Kepler-78 b, Kepler-10 b, CoRoT-7 b, HD 80653 b, 55 Cnc e, and Kepler-36 b. While the aforementioned lists are by no means exhaustive, we believe that candidates presented here can serve as useful input for target selection for future surveys and for testing the importance of core-powered mass-loss in individual planetary systems.

Evidence of energy-, recombination-, and photon-limited escape regimes in giant planet H/He atmospheres

Hydrodynamic escape is the most efficient atmospheric mechanism of planetary mass loss and has a large impact on planetary evolution. Three hydrodynamic escape regimes have been identified theoretically: energy-limited, recombination-limited, and photon-limited. However, no evidence of these regimes had been reported until now. Here, we report evidence of these three regimes via an analysis of a helium I triplet at 10 830 Å and Lyα absorption involving a 1D hydrodynamic model that allows us to estimate hydrogen recombination and advection rates. In particular, we show that HD 209458 b is in the energy-limited regime, HD 189733 b is in the recombination-limited regime, and GJ 3470 b is in the photon-limited regime. These exoplanets can be considered as benchmark cases for their respective regimes.